The present invention relates generally to waveguide circulators and more specifically to wideband junction circulators.
Circulators are microwave components with three or more ports which transmit microwave energy from a first port to a second port while leaving a third port and other additional ports isolated from the flow of energy. The typical waveguide junction circulator is a metallic structure which contains three coplanar waveguides intersecting at the center of the structure forming a waveguide junction. A ferrite rod or triangular prism is mounted at the center of the junction and is subject to a transverse DC magnetic field generated by two permanent magnets. Interaction of microwave energy with the ferrite causes circulation, that is, energy entering the function from one port is directed to the adjoining port in the clockwise direction (counterclock for reverse magnetic bias field).
Many circulator junctions can be combined in one housing to generate multi-junction circulators. The single junction circulator is generally referred to as a three port. A three port with straight waveguide sections is said to have a Y shape. The most common shape is the T shape which has two in-line terminals and a terminal spaced 90 degrees to the in-line direction. The in-line terminals require 30 degree waveguide bends between terminals and junction.
Circulators include waveguide transformers at the junction and extending into each of the channels toward the ports on the top and bottom walls. The impedance transformers match the impedance of the waveguide to the impedance of the ferrite.
The performance of a circulator is measured in terms of insertion loss, return loss, port isolation and operating band width. Performance is considered good when the device has a low insertion loss, high return loss and high isolation over a broad band of frequencies. Broad bandwidth typically assures good temperature stability.
In an effort to increase the isolation or reduce insertion losses, it has been suggested in U.S. Pat. No. 3,555,459 to Anderson to reduce abrupt changes in the walls of the channel between the junction and the port such that d.theta./dy is less than d.theta.'/dy'. Although mathematically accurate, these devices are difficult to manufacture. Impedance matching may also be achieved by a paralleling impedance matching steps in each of the channels as illustrated in U.S. Pat. No. 4,496,915 to Mathew, et al. This is also a difficult device to manufacture.
With an ideal circulator, the response functions of port return loss for different ports are identical, so are insertion loss and isolation for different port combinations. Actual circulators have electrical asymmetries caused by mechanical asymmetries due to the tolerances of the components and the assembly.
Most present art designs require tuning after assembly to achieve the required performance. Typically, dielectric or metallic blocks are cemented into the waveguide sections of the circulator. The tuning is required to correct design deficiencies (systematic errors) and compensate for mechanical asymmetry such as chipped corners or misalignment of the ferrite (random errors). Such tuning is "cut and try", and is therefore, time consuming and too costly a process for mass production.
Thus, it is an object of the present invention to provide a circulator which eliminates tuning to correct systematic errors.
Another object of the present invention is to provide a circulator having design characteristics which are easily and accurately manufacturable.
A further object of the present invention is to provide a circulator of a construction which assures minimum asymmetry.
A still further object of the present invention is to provide a circulator construction which minimizes random errors.
These and other objects are achieved by using a unique shaped transformer having triangular apexes extending symmetrically into the respective channel and connected to two opposed side walls by coplanar tabs. An additional important feature is that one of the channel sidewalls has a minimum of three sections to provide a transition between a first linear section extending from the center junction, and a second linear section extending from the port. The opposed sidewall has only two linear sections. The coplanar tabs are connected to the triangular apexes by a portion curved in the coplanar plane. Where the circulator has three ports and three channels, there are three tabs. The triangular apexes have common linear sides with adjacent triangular apexes and the tabs connect the common side to the opposed sidewalls of the channels.
One of the channels includes two opposed linear walls extending between the center and the port with a pair of opposed studs extending from each wall toward each other. These studs are coplanar with the tabs and are used for tuning. The studs also have a portion curved in the coplanar plane.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.